The present invention provides a method for regenerating mixtures of chromic and sulfuric acids that are used for etching plastics prior to metallic plating. Additionally, this invention furnishes a unique method for substantially recovering and regenerating etchant chemicals which are commonly lost to rinse waters associated with plastic etching processes.
In the practice of plating on plastic, it is frequently required that the plastic substrate be chemically etched or deglazed prior to plating operations. In this manner, the hydrophobic nature of the plastic is reduced and a roughened surface is provided which promotes better adhesion of metal deposits. Although many mechanical and chemical means for depolishing plastics are employed in the art, it is commonly recognized that an aqueous solution containing primarily chromic and sulfuric acids can serve as a general and suitable etchant for plastics such as acrylonitrile-butadiene-styrene (ABS), polyethylene, polystyrene, polypropylenes, phenolics, epoxies, polysulfones, acrylics, polycarbonates, phenylene oxides, and others. The chromium trioxide content usually ranges between about 1 and about 30 percent by weight in these chromic-sulfuric acid etchants. Hexavalent chromium compounds other than chromium trioxide, such as potassium dichromate, may be used to establish the Cr.sup.+6 concentration.
In some systems additional reagents, such as phosphoric acid, also may be included in this type etchant to aid in the production of certain desired surface qualities; but, in all cases, the activity of these various etchant formulations depends upon the oxidative attack of the plastic by hexavalent chromium and the ability of sulfuric acid to solubilize the polymeric materials. However, in a given bath, the etching capacity is soon attenuated by the accumulation of dissolved organic materials and trivalent chromium. The organics reside as fractured units of the original polymer and the trivalent chromium results from oxidation-reduction reactions that occur between hexavalent chromium and the plastic substrate. Typically, when reduced etching activity is noted, or when these impurities reach a certain predetermined level, a process solution of this type is discarded in favor of a fresh bath.
Throughout the industry, it is recognized that dumping of such etchants is extremely wasteful, as the spent solution may contain as much as 70 to 90 percent of the original hexavalent chromium content. Not only does the loss of these values contribute significantly to the overall cost of the etching process, but dumping also adds to this cost in that these etchants present a substantial waste treatment problem. At a minimum, the highly toxic hexavalent chromium must be reduced to the trivalent form and, thereafter, removed from solution. Commonly, trivalent chromium is removed from solution by addition of alkalies to form a voluminous precipitate of chromium hydroxide which requires dewatering before ultimate disposal on land. Substantial losses and waste treatment requirements are created also due to dragout of etchant chemicals from the process bath into subsequent rinse water. Simple attempts to recover these chemicals by evaporation of rinse waters and reuse of residual chemicals in the process has limited value as this may serve to concentrate and return the detrimental impurities.